The present invention relates to a vertical diffusion furnace, and more particularly to a vertical diffusion furnace having improved flow of hydrogen gas.
Vertical diffusion furnaces are rapidly becoming the furnace of choice in many technology areas including the manufacture of semiconductor devices. In contrast with a conventional horizontal diffusion furnace, the vertical diffusion furnace has several major advantages. For example, in furnace applications directed to the fabrication of semiconductor devices, no cantilever or soft-landing is required since silicon wafers are held in a quartz boat which does not touch the process tube walls. Vertical diffusion furnaces allow automatic loading and unloading of wafers. Finally, vertical diffusion furnaces create less of a floor space footprint in the clean room.
One problem which must be addressed in any diffusion furnace application directed to semiconductor fabrication processes is the presence of electrical charge on the boundary region of a silicon substrate and/or an oxide layer formed thereon. Such "unbounded" or stray electrical charge result in degraded performance characteristics for the semiconductor device, such as increased contact resistance in wiring or increased leakage current. In order to solve the problem of stray electrical charge, hydrogen gas is used during alloy processes in the conventional vertical diffusion furnace. The hydrogen gas, which enters the vertical diffusion furnace at normal temperature, rapidly heats in the high temperature environment of the furnace. The energy absorbed during the heating process causes the hydrogen gas to shed hydrogen ions. Hydrogen ions react with uncombined electrical charge present on the boundary region of the wafers and/or one of the intermediate layers disposed thereon to eliminate the stray charge.
FIGS. 1A and 1B show a conventional vertical diffusion furnace. Within the drawings referenced in this application, like reference numerals indicate like elements.
Referring to FIGS. 1A and 1B, the conventional vertical diffusion furnace includes a gas injector through which hydrogen gas is injected into the interior reaction environment of the vertical diffusion furnace together with nitrogen gas. More particularly, the conventional vertical diffusion furnace comprises; a quartz tube 1 forming the interior reaction environment, a plurality of wafers 3 vertically loaded into quartz tube 1, a support 5 supporting loaded wafers 3, and a gas outlet 9 discharges the gas from quartz tube 1 following reaction.
Additionally in FIG. 1A, a gas injector 11 is vertically disposed along the outer wall of quartz tube 1. Hydrogen gas and nitrogen gas are injected into the vertical diffusion furnace through a connector placed in the top of the vertical diffusion furnace. Alternately, as shown in FIG. 1B, a gas injector 11a is vertically installed along the inner wall of quartz tube 1. In both conventional embodiments, hydrogen gas is injected into the quartz tube through gas injectors (11, 11a) together with nitrogen gas. The injected hydrogen gas flows from the upper portion of the quartz tube, over the upwardly loaded wafers, and down to the lower wafers.
Typically, the temperature of the vertical diffusion furnace is about 400.degree. C., and the temperature of the injected hydrogen and nitrogen gases is much lower. This temperature difference causes the injected gases to flow downward within the quartz tube. Additionally, the hydrogen gas and nitrogen gas flow downward in response to gravity. The combined downward thrust of the injected gases generates significant convection throughout the vertical diffusion furnace. The swirling effect of convection on the injected gases inhibits the reaction between the hydrogen gas ions and the loaded wafers. As the result, desired affect of the hydrogen gas, i.e., bonding with and thereby eliminating stray surface charge, is not achieved.
Also, the use of gas injector (11 or 11a) which transports hydrogen and nitrogen gases to the upper portion of the quartz tube increases the manufacturing cost of the furnace. The movement of the gas injector during maintenance operations on the quartz tube may produce particles from the tube which will ultimately contaminate the wafers and reduce yield of the semiconductor devices formed on the wafers. Finally, the elongated gas injector is maintenance intensive.